Steam digestion of impregnated chips



United States PatentOfiiice 3,308,011 Patented Mar. 7, 1967 3,308,011 STEAM DIGESTION F IMPREGNATED CHIPS James H. Ross, Montreal, Quebec, Canada, assignor to Beloit Corporation, Beloit, Wis., a corporation of Wiscousin No Drawing. Filed June 20, 1963, Ser. No. 289,436

' 1 Claim. (Cl. 162-68) The present invention deals with a process for recovering cellulosic pulps from a fibrous vegetable material and, more particularly, to an improved process for the recovery of higher quality pulps from fibrous vegetable materials using standard chemical pulping reagents.

The most prevalent chemical pulping process employed today is still the sulfate or kraft process, followed in usage by the su-lfite process.

In the kraft process, the process steps can be broken down into three categories: a penetration and pressureraising period; a full reaction period; and a gassing-down period. In the first period, the air is removed from a digestor, and the cooking liquor consisting of sodium hydroxide and sodium sulfide penetrates the chips. The more soluble constituents such as the hemicelluloses begin to dissolve at this stage, and the volatile terpenes and oils begin to distill off. When the steam pressure reaches about 40 to 60 pounds per square inch, sulfide and mercaptan compounds of lignin start to form as a result of the reactions with sodium sulfide. These compounds are more readily soluble in soduim hydroxide than unreacted lignin and therefore the lignin goes into solution rapidly.

While the aforementioned process is a reasonably rapid one and produces a strong pulp, it also causes substantial removal of carbohydrates by solubilization, thereby reducing the yield possible.

One of the objects of the present invention is to provide a chemical pulping process which is more eifective in removing lignin from wood than conventional methods.

Another object of the present invention is to provide an efficient chemical diffusion process for introducing pulping liquor into wood while diminishing the usual solubilization of carbohydrates.

Still another object of the invention is to provide a chemical pulping method which produces a higher yield of pulp, with a smaller percentage of rejects.

Still another object of the invention is to provide a method for the impregnation of wood which results in a less contaminated liquor so that it may be reconstituted and reused more efiiciently.

Another object of the invention is to provide a wood impregnation and diffusion process which can be employed at atmospheric pressure.

In accordance with the present invention, I impregnate fibrous vegetable material such as wood chips with a cooking liquor, and then drain the excess cooking liquor from the material after the impregnation. The impregnated material is then held in the absence of external liquor until the liquor contained in the interstices of the wood diffuses through the wood. This holding or diffusion step is carried out at a temperature sufficient to cause thorough migration or distribution through the wood without, however, causing significant pulping of the wood material. Finally, the temperature of the impregnated wood is raised by means of contact with steam to cause pulping of the material.

My process is particularly applicable to the vapor phase digestion of wood since it insures that the wood has a sufiicient chemical content for efficient cooking.

Since the wood and liquor undergo a changing equilibrium as the liquor is consumed by the wood, it is important that a good chemical distribution be achieved before vapor phase digestion can be carried out. I have found that the very rapid apparent equilibrium which takes place between the chemcial treating agent and the wood even at temperatures close to and below the boiling point of the liquor, if carried out in a liquor of appropriate strength, places in or on the wood'sutficient chemical to finish the digestion and leave a liquor of proper strength in the wood for proper solubilization of the lignin without damage to the fibre or undue solubiliz-ation of carbohydrates.

Turning now to the specific details of the process, the Wood chips or other fibrous vegetable material are impregnated, in the case of a kraft type process, with a sulfide liquor having an effective alkali concentration of about 50 to grams per liter. The impregnation takes place at atmospheric pressure and at a temperature from about 80 to C. Then, excess cooking liquor is drained from the material and the material is held at atmospheric pressure for periods of time ranging from about 10 minutes to 2 hours until it is thoroughly diifused through the material. Temperatures of from about 120 to C. are appropriate for this diiiusion step. Under these conditions, the rate of chemical reactivity is relatively low, but the chemical distribution rate is relatively high. Finally, the impregnated wood material is cooked by means of contact with direct steam at temperatures normally ranging from about 160 to 185 C.

The following specific examples illustrate the manner in which the invention may be carried out.

Example 1 A mixture of standard size western softwood (mostly Douglas fir and ponderosa pine) chips were charged into a pressure vessel. Steam at atmospheric pressure was passed through the chips for 10 minutes to remove most of the air from the vessel. An impregnatnig liquor having an effective alkali concentration of 70 grams per liter and a sulfidity of 30% was circulated through the bed of chips for 20 minutes at 100 C., at atmospheric pressure. At the end of the 20 minute impregnating period 17.9% effective alkali had been adsorbed by the chips, based on the bone dry weight of the chips. All of the free liquor surrounding the chips was drained away and saturated steam -at a pressure of 45 p.s.i. gauge was introduced into the vessel to raise the temperature of the impregnated chips to 145 C. The chips were held at this temperature for 45 minutes to allow enough time for the chemicals to diffuse into the innermost fine structure of the fibers. Following the diffusion period, the temperature in the vessel was increased to a maximum of 178 C. with direct steam. This temperature was held for 40 minutes to complete the digestion. The result-ant pulp was washed, screened through a 0.010 inch slotted plate, and evaluated for yield, strength, and lignin content. Its screened yield was 49.6%, and only 1.5% rejects were obtained. The pulp had a lignin content evidenced by a Roe chlorine number of 13.0.

In comparison, pulp produced at the same level of Roe number by the use of a conventional sulfate procedure which consisted of increasing gradually the temperature of the chips submerged in liquor (34.1 grams per liter efiective alkali concentration) from 50 to C. over a period of 90 minutes and then holding the maximum temperature for 40 minutes had a screened yield of 47.0% and produced 50% more rejects. The trength properties of both pulps were substantially similar.

Example 2 The same time and temperature schedule as in Example 1 was used, but the impregnating liquor concentration was increased to 80 grams per liter of effective alkali in order to reduce the lignin content of the pulp. In this case, after 20 minutes at 100 C., the chips had adsorbed 20.6% of effective alkali, based on the bone dry Weight of the Wood. The pulp produced had a screened yield of 46.4%, gave 0.1% rejects, and had a Roe number of 8.8. Conventional pulp having the same Roe number gave a screened yield of 44.7% and had 1.4% rejects.

With the use of the process of the present invention, the liquor used in the impregnation-diffusion stage is less contaminated with dissolved Wood product and so may be readily strengthened and reused. The absence of external liquor during the diffusion stage, and the relatively unreactive conditions prevailing during this stage prevent undue reaction between the 'WOOd and one of the ions in the solution which might alter the overall reactivity of the lignin. It is known, for example, that the sulfur bearing ion in kraft liquor distributes more slowly than the sodium ion, but for good pulping it must react With lignin to make it soluble which it might not do if the lignin had already reacted with the sodium ion.

It should be evident that various modifications can be made to the described embodiments Without departing from the scope of the present invention.

I claim as my invention:

The method of recovering cellulosic pulp from Wood chips which comprises charging the chips into a pressure vessel while displacing the air therefrom with steam, circulating with said chips 21 sulfide liquor having an alkali concentration of to grams per liter for 10 to 30 References Cited by the Examiner UNITED STATES PATENTS 1,347,979 7/1920 Werner 162-69 X 2,190,194 2/1940 Richter 162-69 X 2,858,213 10/1958 Durant 162-237 2,999,784 9/1961 Hullmann 162-49 3,165,436 1/1965 Bennett 162-69 X DONALL H. SYLVESTER, Primary Examiner.

HOWARD R. CAINE, Examiner. 

